Project Summary - Siderophore-dependent inhibitors of Mycobacterium tuberculosis Mycobacterium tuberculosis (Mtb) is the leading cause of human deaths from an infectious disease. The astronomical costs of treating tuberculosis patients infected with drug-resistant Mtb42and the very long treatment times pose a huge financial burden on health care systems in particular in developing countries. Even more challenging is the increasing number of infections with extensively or totally drug-resistant Mtb that have become untreatable. Thus, the development of new TB drugs, preferably against new drug targets, is urgent. Iron is acquired by Mtb by secreting siderophores, small molecules with high affinity for iron called mycobactins and carboxymycobactins and subsequent uptake of iron-loaded siderophores. We discovered that genetic inactivation of the siderophore secretion system in Mtb leads to intracellular accumulation of siderophores and self-poisoning of Mtb reducing its virulence in mice by more than 10,000-fold. Based on this observation we have developed an innovative, whole-cell screening assay to identify compounds which inhibit Mtb in a siderophore-dependent manner. Screening of ?300,000 compounds and subsequent structure-activity relationship (SAR) studies yielded four lead compounds with exciting properties (novel mechanism of action, nanomolar activities against Mtb, low toxicity and strong synergy with some TB drugs. These preliminary results validated siderophore secretion as a novel and druggable target of Mtb. The aims of this proposal are to examine the activity of siderophore-dependent inhibitors in a mouse model of tuberculosis, to determine their activity against drug-resistant Mtb and to identify their molecular targets.